METAZOA

Question 1

Evolution of multicellularity

Answer 1

1) Form a composite multicellular organism (different functions)
2) Asexual divisions remain together after fission
3) Multinucleate protist evolves internal membrane partitions

Question 2

Metazoan body plan

Answer 2

1) multicellular bodies
2) heterotophic nutrition
3) cell differentiation
4) nervous system
5) move using muscle contraction

Question 3

Why be multicellular?

Answer 3

1) division of labour via cell differentiation

2) larger size allows protection from abiotic/ biotic environments and control of internal environments

Question 4

Multicellular characteristics

Answer 4

• distinguish self from non self
• stick together
• communicate
• exchange metabolites

Question 5

Multicellularity is ‘easy to evolve’, but what is the underlying issue in the transition from protists to true animals?

Answer 5

True animals are way more complex

Question 6

Candidates for the ‘simplest animals’ (with characteristics)

Answer 6

1. Sponges (Phylum: Porifera) questionably metazoan, occasionally regarded as a colony of protists: no nervous system/ no tissues/ no nervous system
2. Placozoans, basically a multicellular amoeba
3. Comb Jellies (Ctenophora) have 3 cell layers and a complex nerve net
4. Anemones (Cnidaria) have 2 cell layers and a simple nerve net
5. Flatworms (Platyhelminthes) have 3 cell layers and a complex nervous system

Question 7

How do we link simple ancestors with the simplest animals?

Answer 7

1) Fossil evidence - most early fossils cannot be linked to modern groups (left no descendants) eg. Charnia ca 550-600 Mya
2) comparative evidence (choanoflagellates are v similar to sponge filter feeding choanocytes)

3) developmental evidence -The same HOX system is shared by all metazoan groups including sponges, but is
absent from protists - a crucial evolutionary breakthrough?
Developmental evidence strongly supports a SINGLE
ORIGIN for all animals.

4) DNA sequencing evidence- difficult to find a gene that has evolved slowly enough to retain differences

Question 8

Aspects of morphology that are shared by all metazoans,

but are ABSENT from all protists

Answer 8

• Neurons of nervous system.
• Cell-cell ʻtightʼ junctions.
• Striated muscles.
• Collagen
• A specific form of an enzyme, tyrosine kinase

Question 9

Metazoa are…

based on DNA phylogenies

Answer 9

• monophyletic (derived from a single ancestor)
• > 1bn years
• closest to choanoflagellates
• related basally by Ctenetophores and therefore 3 cell layers must have evolved independently twice
• sponges and placozoans may be SECONDARY SIMPLE ( loss of ancestral nervous system)

Question 10

What does paeodomorphically mean in the context of animal evolution?

Answer 10

• several groups bar resemblance to the larval stages of others

Question 11

Sponges are the closest of all animals to be regarded as a ___ ___ ___ rather than a multicellular animal.

Answer 11

colony of protists

Question 12

Why can’t Charnia fossils be linked to modern groups?

Answer 12

• complexity jump too great, can only be linked to rapid diversification

Question 13

Free living choanoflagellates are v similar to __ __ feeding cells.

Answer 13

sponge

filter

Question 14

Issue with DNA sequence comparison

Answer 14

hard to find gene that has evolved slowly enough to retain differences